1. Field of the Invention
The present invention relates to RF receivers, and more particularly, to direct conversion television tuners and receivers.
2. Description of the Related Art
Direct-conversion, or homodyne, receivers are today popular for many communications applications because of their simplicity and low power. They do not require intermediate-frequency (IF) filters, which are often costly, and need only one frequency conversion stage and one local oscillator (LO). Direct-conversion receivers rely entirely on quadrature mixing to obtain the necessary image rejection. The conversion gain and phase of the quadrature mixing stages must be precisely matched to obtain good image rejection.
At the frequencies used for direct broadcast satellite (DBS) transmissions (approximately 1-2 GHz), 40 dB of image rejection is about the maximum level of image rejection that can be achieved solely through component matching in practical integrated circuits.
This is adequate for the quadrature phase shift keying (QPSK) and 8-ary phase shift keying (8PSK) modulation formats used in these systems. However, analog television requires signal-to-noise ratios of about 50 dB for ideal image quality. As there are many contributors to the noise budget in realistic communication systems, the noise contribution from images must be on the order of 60 dB below the signal or better.
FIG. 1 shows a conventional direct conversion analog television tuner. As shown in FIG. 1, RF input is received by a low-noise amplifier 101. The RF input is then fed into mixers 102a, 102b. Phase-lock loop (PLL) 105 outputs a waveform at a specified frequency to the mixer 102a, and, through a 90-degree phase shifter 104, to the mixer 102b. Collectively, the mixers 102a, 102b, the PLL 105 and the phase shifter 104 may be referred to as a mixing stage 125.
The outputs of the mixing stage 105 are fed to low-pass filters 106a, 106b, and then to variable gain amplifiers 108a, 108b. The variable gain amplifiers 108a, 108b are in a closed loop that includes power detection circuits 109a, 109b, respectively. The outputs of the amplifiers 108a, 108b are the quadrature components I and Q, respectively.
The analog television spectrum contains a discrete tone at the picture carrier. This picture carrier will appear in the I and Q signals at the same frequency. Ideally, the I and Q picture carriers will be equal in amplitude and 90 degrees apart in phase. However, because of circuit imperfections, the I and Q picture carriers will have slightly different amplitudes and be somewhat more or less than 90 degrees apart. This is illustrated in the spectrums of FIG. 2.
As may be seen in FIG. 2, the I and the Q components are unbalanced. In this case, the amplitude of the Q component is larger than the amplitude of the I component by δ, and is out of phase with the I component by φ.